Wednesday, May 2, 2007

In Trees--what about them? I mentioned that not all trees are created equal in their ability to promote or discourage warming. The Economist writes about a daring computer modeler who has taken on the question, "What if we just get rid of all of them?" The assumption by many AGW promoters

that planting trees will make the world cooler than it would otherwise be, is the subject of a newly published study by Govindasamy Bala, of the Lawrence Livermore National Laboratory, in California, and his colleagues. Dr Bala has found, rather counter-intuitively, that removing all of the world's trees might actually cool the planet down. Conversely, adding trees everywhere might warm it up.

Dr Bala's model can be told to replace all the world's forests with shrubby grasslands, and left alone to work out how such a change would alter greenhouse-gas concentrations and how that, in turn, would influence the temperature in different places.

When Dr Bala ordered global clearcutting, the model calculated that the atmosphere's carbon-dioxide levels would roughly double by 2100. This is a much greater increase than happens in a business-as-usual simulation, but it would, paradoxically, make for a colder planet. That is because brighter high latitudes would reflect more sunlight in winter, cooling the local environment by as much as 6°C. The tropics would warm up, since they would be less cloudy, but not by enough to produce a net global heat gain. Overall, Dr Bala's model suggests that complete deforestation would cause an additional 1.3°C temperature rise compared with business as usual, because of the higher carbon-dioxide levels that would result. However, the additional reflectivity of the planet would cause 1.6°C of cooling. A treeless world would thus, as he reports in the Proceedings of the National Academy of Sciences, be 0.3°C cooler than otherwise.

If indeed the fate of civilization depends on fractions of a degree, we had better sharpen up the axes and get rid of all those trees. If higher carbon dioxide levels are important only because they lead to warming, but we can have less warming and more CO2 at the same time, that would lead to better crop yields in the regions that are currently under cultivation. Unless the crops were apples. Or timber. But you wouldn't need to harvest any more timber, because you would already have harvested it all. But there would never be any more.

I like trees pretty well. I think we should keep them. Some, at least. The good ones. Treebeard has a list.